I am just about to start construction of a small (100 square metres) single storey timber frame house.The posts will be freestanding on stones,and the timber frame will be 105mm x 105mm throughout.
I have never built this sort of structure before,and I have a few specific questions related to the mortise and tenon joints.
1.I intend to pin all joints.With the through tenons I have no query,but there will also be stub tenons,where two tenons butt each other through the post.These stub tenons will therefore be half the width of the post,so 52mm.My query here is: I understand there should be 3 times the width or diameter of the pin left in the tenon,after the pin.I calculate that this would allow me a pin of not more than 10mm diameter,with a length of 105mm (so as to leave 30mm of tenon after the pin,and 12mm of tenon between the pin and the shoulder).The proportions of the pin seem fairly thin,for the size of the timber?
2.Further to that - any views on the relative merits of square versus round pins?
3.In my plans I have vertical through tenons extending from the top of some posts,onto which would be fixed a beam,with the other end of that beam being fixed into a post by means of a regular one third width rectangular horizontal stub tenon.The query in this case is: in the case of a vertical tenon,it seems logical to me to use a square centred tenon of 50mm x 50mm dimensions (105mm length)...........?

1. An alternative to two tenons butting in the middle of the post is to make an over and under tenon. This way they can be full length and have plenty of wood left for the pin diameter problem you are facing.
2. round pins are easier to bore a hole. square pins would require more work to make exact sizes.
3. You can center the tenons if you want to, but you need to consider the layout on both pieces, so that they align properly.
We use a method called "square rule" where you layout by using a standard framing square and offset the tenon by either leg of the framing square that way the layout is the same on both pieces as you use the same tool to layout both.
And you need to consider the amount of wood between the mortise in the beam and then end of the beam. It is usually stronger to leave some small section of wood between the mortise and the end of the beam so you wouldn't have a open mortise on the end.

There are good views of joint in the pdf files available from the guild website for free download.
Look for the Historic American Joinery series and you can view different joints, together and exploded to see how timbers can be joined.

Good luck and keep asking questions.

Jim Rogers
PS see over and under tenon example in my avatar, to the left.

For house framing, I believe the cross section of your members is far to small, my comfort level would increase greatly if the timbers would double in size.

Hello,

In the plan you have laid out you describe two tenons coming together and butting against each other mid way in a through mortice but you leave no room to accommodate for shrinkage and expansion of the post. The effect will be that the tenons bottoming out against one another will force a gap at each shoulder half the width of the amount of shrinkage or if pinned possibly even worse.

Your thinking on the pegs is correct and with that in mind but also the extra effort involved in the peg openings, the octagonal or six sided peg in a round hole is the best compromise solution.

Greetings,

Don Wagstaff

Thanks for the informative replies everybody.
An over and under tenon looks interesting,although at this stage I don't have the confidence in my woodwoorking skills to start considering more complicated joints.I'd prefer to keep the joints as simple as possible - as long as my dimensions allow for pinning.As for the timber dimensions (105mm x 105mm),these are the standard dimensions for posts in the part of the world where I live (Japan),and since our house will only have a light,low-pitched tin roof I feel no need to increase the beam dimensions.Things here are generally done on a smaller and finer scale than in the West,something that it's taken me a while to get used to.
Likewise the pin shape here is usually square,which probably reflects the fact that yesteryear's builders had no access to powerdrills - and neither will I.I'll be doing everything with hand tools,with no access to a power source.
The alignment of the square tenons with the ones on the other side of the beam is a point to consider,although I'm a little stumped by the meaning of the terms "square rule" and "layout"......?
Also the shrinkage of the posts and consequent exposure of the mortise shoulders is something to consider.I wonder if it would be worth shortening the blind tenons a little - although that would of course exacerbate my problem with the pins,which remains my main concern.
In case anyone is wondering why I'm asking these questions on this forum,when I live in a land that contains the oldest and most intricate wooden structures in the world - well,it's partly a matter of limited language skills on my part (this is not a simple subject),as well as the cultural difficulty of obtaining a direct,to-the-point answer in this part of the world.But most importantly it comes down to the fact that pure timber framing is a dying art here,and it is very difficult to find one of the few remaining old men whose craft it is,or was.

Regards
Iain Andrews

Could you explain the effect an octagonal peg in a round hole would have Don? (Please excuse my ignorance!)

Regards
Iain Andrews

When making your own pegs with out a lathe the end result is an eight sided peg. It is not necessary to shave off the eight corners making it nearly round, that would be superfluous. So, I would say that the corners are not the goal. The goal is to fashion a working peg, any farther beneficial benefits are gravy. The eight corners, compared to turned round pegs, do not hold any negative side effects. The peg should be tapered as well to aid in draw boring.

A square peg in a round hole will cause undue force on the tenon. It could crack it.

Hello,

Also because in Japanese timber building a round(ish) peg is thought of, (as it is), as inferior in both function and appearance.

Well not only is the 8 sided peg easier to make properly but it will always fit as there is a degree of cutting action. With a properly fitting round peg as you can imagine the fit is 100% compression and that requires more force to achieve the same surface contact. It's the same with round or wire nails and lets say cut nails - being also tapered - but because they cut through the wood fibers instead of pushing their way through they create the maximal surface contact with intact wood fibers and so grip better along the entire length.

Also a rounded off opening exposes a maximal amount of end grain, undesirable in any case but on the verge of offensive in Japanese woodworking.

An allowance for wood movement is also well accounted for in Japanese timber construction. Tenons never bottom out in a mortice. In Japanese joinery books I think a 5mm space is allowed for.

Greetings,

Don Wagstaff

Don:
I agree with many of your points, but I'm not following you on this one:

"Square rule" is the name of a method of marking lines on a timber so that all the joints line up from one timber to the next.

In Japan I believe they use center line layout.

Layout means drawing lines on the wood to represent the tenon/mortise and waste wood to be cut away.

We usually layout the lines first so that we can see what will be cut away and what will stay.

I can understand your need to do things with only hand tools.

To make pegs you can use a draw knife if you have one. And you can build your own simple shaving horse to hold the stock while you shave it to size and shape.

If you don't have a draw knife then you can use a plane or a chisel. It's a bit harder to do, but you have to use the tools you have on hand.

To bore holes without power drills you need to find a "T" handle auger bit. It is a simple but effective way to bore holes for removing wood from mortises or drilling peg/pin holes.

Jim Rogers

Hello Clark,

A mortice could be one example of an opening that I'm meaning, so cutting it necessarily exposes end grain at the top and bottom generally and since the pathway an arch makes is longer than a straight line a pill shaped mortice degrades the wood more or exposes more end grain than a comparable rectilinear mortice. This is one reason that in Japan square pegs are preferred. In a glued joint, so certain types of furniture construction, a rectangular mortice provides more glue surface than a mortice cut by say a router or a slot borer with rounded short ends where a rounded off tenon would be inserted.

Greetings,

Don Wagstaff

Hello,

Don't know what happened to the one that got this topic going. Maybe we were just being put on, maybe it was one of those smart junk mail programs but the peg subject did get me thinking and once thought through I convinced myself to the merits of the square peg over round and so made the application in some current work.






Well, I am pretty pleased all around with the effort and find it well worth the extra time put in.

Greetings,

Don Wagstaff

I have also pondered the peg, thinking it acts both as a riving wedge, due to variable abutting surface, and a key.

Hello,

Huh? But a key is the opposite of a wedge!

Greetings,

Don

I ponder if the square peg in a true tension situation would enhance the relish to shear out easier. Due to the two sharp corners making nice starts for the relish to blow out. I picture a round hole confuses the stresses and can't find a good starting point. Or at least not two clear starting points, only one from the center of the round hole. A similar effect is seen in curves cut with an adze on the under sides of joist, spreading out the shear point, as opposed to a square reduction or even a 45.

Hi Don, a normal square key abuts force at 90 degrees but if the key is rotated 45 degrees the reaction is both as a key and a riving wedge.

Roger, I do agree.

But now we'd have to properly align a square peg hole to the proper position to act as a wedge and a key. And this square peg hole as to be cut accurately for the square peg to act as a wedge and a key.

In my opinion that would make so much more extra work.

I was once told: "there is a whole world in a single blade of grass..."

The point of using a round peg and in round hole is to secure the tenon to the mortise is quick and easy. Let's do this and be done with it.

I think we're splitting hairs here.

Jim Rogers

Tim, rather than confusing the forces, why not consider the reaction forces with a peg and a bore shape the the reaction into a 180 degree spectrum.

I agree with Jim. But for fun could our elaborate on this Roger? Rotating the square peg allows for 3 point to blow out. The two sides and the center.

I am about to cut some long tenons on the bottom of two kings, they will be wedged with half round wedges and single round hole as the keying mechanism. This will be replacing a squarish tapered wedge on the underside of the tie. Much simpler to cut all around. 2" rived peg stock split down the middle for the wedge.

I have always believed in making joinery such that pegs are unimportant beyond securing things together for raising. I know many fine carpenters use pegs for structural reasons, but I avoid this at all times. And Tim introduces part of why I do so. To reduce the tension force of entire timbers down to one or two much smaller points can create a tremendous pressure. To me it seems better to have the joinery itself designed to handle tension if need be (yes I understand there is the incident of live forces temporarily placing a compression joint into tension as well)

I have always been a big fan of peg-less joinery, believing the fewer holes you cut in the timber, the better.

It's the word key that throws me here. I'm thinking key as in key stone, absorbing pressure from two sides co pressure, com-pression. It could be that I'm not familiar enough with your timber framing jargon in this case.

As for splitting, I don't think it's an issue of the peg, round or not. It is another matter altogether, could be in the area of accuracy and appropriateness of the joinery, something like that. Take the simple example of hammering a nail into a board. If I hammer a relatively small nail close to the end of a far thicker board of say spruce it's likely to split that board. At the same time, if I clamp a thin strip of lattice, of the same wood, in a vice, (I could just as well pre-drill for the nail. Or don't forget blunting the tip, that one works wonders, or even lubricating the nail, sometimes I use that one too), I can hammer a big nail just as close to the end take it from the vice and there will be no weakening of the wood relative to the nail. If I split the wood then the split is just as likely to go around the nail/hole. In other words it's the wood structure influencing the split not the nail or peg in our case.

I think there is a notion of the mechanics of the joinery and the structure and properties of the wood going on here that may be intuitive but is incorrect.

Another thing is the matter of which looks better, round-like or square-ish. And to add to that, in my examples up there the openings for the pegs are off-set, so some drawn effect taking place on those. I was surprised that there was no splitting with them.

Greetings,

Don Wagstaff

Don, from the TF Glossary

KEY. Small element, usually wedge-shaped, used to lock
a joint or, if a shear key, to prevent sliding of one member
over another.
KEYED BEAM. Two or more beams laminated together
with crossgrain keys let in between the beams to prevent
slippage during bending.
SHEAR BLOCK. Wood block dapped (q.v.) partially to
adjoining parallel laminae in a built-up chord, designed to
resist shear between the two members or to transfer load
around a discontinuity such as a scarf, and properly oriented
parallel to the grain, so that shear block end grain bears
upon chord end grain.
SHEAR KEY. Wood block oriented perpendicular to
(across) the grain. Easier to assemble, and can be tightened
if wedge-shaped, but not so resistant to compression
as a shear block (q.v.).

In my usage a key has a flat surface oriented square to the force, except as a sloping shear block. The example of the rotated key is just a simple visual example of how sloping surfaces can impart force, a peg has a sloping surface that varies from parallel to square to parallel from the direction of force.

So, how does riving force matter? In a quiet frame not so much, the pegs function as place holders however in a tension generating frame with open joinery. such as a frame using free tenons, I do have concerns over the chance that the tenon will split. So in that case, square pegs or keys seen like a better choice.

In Swiss architecture, it is very common for sills to be joined together with a keyed double tenon. The tenons pass far out the other side, and are secured in place with a strong key, creating a joint which can move no where.

The preferred practice in this situation is that the key be square and straight, not tapered. This requires the joint to be very carefully formed as you cannot rely on a wedged key to draw it together. But the thought is that it is stronger this way. Also, a straight key does not work itself out over time.

And then, on framed buildings, pegging is often totally absent.

For you reading pleasure, the dissertation "Investigation of Through-Tenon Keys on the Tensile Strength of Mortise and Tenon Joints" by Lance David Shields

http://scholar.lib.vt.edu/theses/available/etd-07112011-100606/unrestricted/Shields_LD_T_2011.pdf

Jim

I only breezed about the pdf, and saw no mention of round keys. I would be very interested in info on round keys. Were they ever used in the past and if not why not?

Split ring connectors would be one instance of a widely used round shear keys. NDS will have useful specs regarding their use.

Pegs are roundish, too. Same effects. In the link Jim provided I noticed nice clear stock for their test, for predictable constancy. Where are the test with knots at strategic locations? Say, the key placed right where a knot is, or have a knot just below the key in the shear plane. Any test with this, how about at any of the guild events where they have busted joints? Results? Is it worth it to take the time to lay out keys on through tenons with knots in mind?

Round keys have also been used between tie and top plates, at the edge of the tie, aiding in withdrawal of the tie from the top plate as thrust is applied to the wall from the rafters. Most likely bored and installed after the structure was assembled.
Seen in HATJ A Graphic Guide, pages 12 and 13, fig. 15,16 &17. They are not labeled as keys but pegs.

Hello,

They've crunched the numbers, but I think they've cooked the books. I'm a bit with Tim on this one, not being such a reductionist particularly in the matter of wood.

Greetings,

Don Wagstaff